1. Field of the Invention
The present invention relates to virtual storage systems supporting a plurality of physical storage devices.
2. Background Art
Computing systems may include one or more computing devices capable of accessing a plurality of physical storage devices. These storage devices may support one or more of a variety of media types including magnetic disk, magnetic tape, optical disk, optical tape, and the like. Computing devices may directly access physical storage. Such an access would typically provide a sequential listing of blocks to be written to or read from the physical storage device. These blocks may or may not be actually stored on the physical device in the sequential order specified. A controller, which may or may not be part of the computing device, interprets the access request and attempts to access the requested blocks.
The controller may encounter several types of errors when attempting to access physical storage. A communication error occurs when the physical storage device does not respond to a physical access request. This may occur because the physical storage device is off-line, is busy, is inoperative, or is otherwise unavailable. An access error occurs if the physical storage device is responding but the requested block cannot be accessed. Access errors may occur because the block is damaged, is deleted, is protected, or the like. Typically, the controller returns a status indication to a requesting computing device. The status message reflects whether the access was successful or not and if an error occurred, the error type. In the case of a read operation, data read prior to the error may also be returned by the controller.
A virtual storage system may be implemented to remove many of the storage details from computing devices accessing stored data. Typically, a controller implementing virtual storage presents the view of a single virtual storage device to the computing device. The controller may physically maintain data for a request placed by the computing device on many physical storage devices of the same or different types. The controller may also provide a variety of support functions such as maintaining volume tables, providing automatic backup and restore services, load balancing, compression, encryption, defragmentation, error detection and correction, and the like.
One problem with virtual storage is in handling error messages. A logical request placed by a computing device to access a virtual storage device may require the access of more then one physical storage devices to fulfill the request. Problems arise with how to handle error messages generated by one or more physical storage devices.